Revise the passage by changing all of the gerunds to infinitives and all of the infinitives to gerunds.
“giving is one of the best things people can do. to volunteer their time shows what caring people they are.”

On the one hand, a gerund is a verb that acts as a noun, know for the -ing form. On the other hand, an infinitive is the basic form of a verb, that usually follows  "to" or another verb form.  The revised sentence, including both of these, is:

"To give is one of the best things people can do. Volunteering their time shows what caring people they are."